Labeljoy normally tells the printer which paper to use: the one in the document's layout. There's one exception. Once you've set the printer up with Printer preferences, in the Print panel, the document keeps the printer's own settings, paper included. Change the layout's paper after that - from Letter to Legal, say - and the two no longer agree.

That's when you see this, as you print:

The document "Shipping labels" is set to print on paper size 8 1/2" × 14", while the selected printer "Office printer" is set to paper size 8 1/2" × 11".
Are you sure you want to proceed?

The sizes are width by height, in inches or in centimeters - whichever the document uses - and they take the orientation into account: the same Letter page turned on its side is a different size.

What you can do
OK	Prints anyway, on the printer's paper. Labels may come out shifted, or cut off at the edge.
Cancel	Prints nothing.
Hide this message	Stops asking, from now on: every print goes ahead on the printer's paper. Restore hidden messages, in the Settings panel, brings the warning back.

To fix it for good, make the two agree again: open Printer preferences and choose the paper that matches the layout, or put the layout back to the paper the printer is set to.

A print started from the command line asks the same question in the console, unless you've hidden it. With nobody there to answer, the answer is no: nothing is printed, and the run ends with an error.